The present invention relates to a system for rapid closure and rapid opening of the fluid-flow cross-section of a duct and to the application of such a system to the rapid closure and rapid opening of the gas-inlet and/or -exhaust ducts in a two-stroke combustion engine.
These engines comprise at least one cylinder in which a reciprocating piston is housed, the said cylinder defining a combustion chamber in fluid communication with a fresh-gas inlet duct and a burnt-gas exhaust duct emerging in the said chamber, respectively, via at least one inlet port and at least one exhaust port, the said ports being capable of being closed by the piston according to its position in the cylinder. In practice, there are generally several inlet ports and a single exhaust port.
Such combustion engines have low thermodynamic efficiency, develop low maximum torque and are a source of pollution.
The cycle of this type of engine can be broken down as follows:
explosion of the gases: the piston descends while the exhaust and inlet ports remain closed by the skirt of the piston; PA1 start of the exhaust phase when continuation of the descent of the piston causes opening of the exhaust port; PA1 start of the inlet phase when continuation of the descent of the piston causes opening of the inlet ports; PA1 end of the inlet phase when the inlet ports are closed following the ascent of the piston; PA1 start of the compression phase of the fresh gases on closure of the exhaust port following continuation of the ascent of the piston. PA1 devices which modify the height of the exhaust port as a function of operating parameters; PA1 devices which modify exhaust tuning as a function of operating parameters, and PA1 devices which manage inlet timing and exhaust timing, defining the start and the end of their respective phases as a function of operating parameters. PA1 small overall size; PA1 good mechanical reliability due to the reduction in the number of moving parts, such as valves, rocker arms, etc., and the dispensing of parts no longer covered by the definition of the engine, such as the cam shaft, timing chain, etc.; PA1 high specific power output; the cycle of this type of engine is twice as short as that of the four-stroke engine so, at the same speed, it executes twice as many power strokes and its power, for equal thermodynamic efficiency, is doubled.
As of this instant and up to closure of the inlet ports following the ascent of the piston, the exhaust and inlet ports are open and in communication; this period may be broken down into two phases: first, there is scavenging of the residual burnt gases by the fresh gases, then filling of the cylinder with fresh gas.
It is the long crossover period during which the exhaust and inlet ports are open which gives the twostroke engine its poor qualities in terms of torque, efficiency and pollution. In fact, a large part of the fresh gases is lost in the exhaust on scavenging, on inlet and during the time gap separating closure of the inlet ports from that of the exhaust port. In the case of engines equipped with an carburetor or an injection system in the inlet duct, the loss of efficiency and the rate of pollution are proportional to the amount of unburnt fuel lost.
Moreover, in this type of engine, compression begins only after closure of the exhaust port, the compression ratio is thus very low and filling at the start of compression is mediocre due to the losses. The engine torque is thus considerably restricted.
Moreover, during each cycle, the power stroke ends on opening of the exhaust port. The engine thus does not take advantage of all the expansion of the gases, it loses in terms of operational flexibility, combustion is aborted and a certain amount of unburnt gases is discharged into the exhaust, further aggravating the poor efficiency and the rate of pollution. It will also be noted that scavenging of the residual gases is incomplete and that the compressed gas mixture exhibits a certain content of gases burned during the preceding combustion. Propagation of the flame during combustion is impeded and thermodynamic efficiency diminished.
Numerous devices have been developed in order to attempt to rectify these defects. The object of all of them is to influence one or the other of the inlet and exhaust patterns so as to reduce losses of fresh gas and to improve filling of the cylinder.
These devices are based on three main principles:
Only those devices which are based on the latter of these three principles constitute a real response to the problem posed. There are numerous innovations in this field, but lack of efficiency means that genuine technical progress remains poor.
GB-A-2,117,047 MONTGOMERY also discloses a system for rapid closing and rapid opening of the fluid-flow cross-section of a duct, consisting of a pair of two rotating parts turning, in opposition, about parallel axes, the said rotating parts each defining a recessed surface of revolution, the said parts being guided in rotation so that, during the rotation of the said parts, their solid parts achieve tangential pseudocontact which totally closes off the duct, whose flow cross-section is then freed by the continuation of rotation of the parts bringing their recesses opposite one another, the said axes of rotation of the rotating parts being perpendicular to the longitudinal axis of the duct at the level of the said tangential pseudocontact.
"Pseudocontact" is understood to mean that a clearance (for example, of the order of 1/10 mm) is retained in order to account, in particular, for expansion of the parts.
The MONTGOMERY rotating parts are "blade" valves. Such blade valves, which are rotating parts comprising, principally, two recesses mutually defining a blade, do not permit asymmetric timing optimizing the pattern, that is to say do not permit opening and closing of the ducts at the right moment and thus do not make it possible to obtain optimum efficiency and reduced pollution.